From fb88a5ce1f5bae1af40b42a2dbf229c621f71e9f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jakub=20Per=C5=BCy=C5=82o?= <jakub.perzylo@swmansion.com>
Date: Wed, 29 Apr 2026 18:59:37 +0200
Subject: [PATCH] Add audio encoder reconfiguration

For some platforms (looking at you Apple) we can't reliably get the
channel count from the media track, this information becomes available
only when looking at the data inside of audio worklet callback.

The following changes allow for config creation only after receiving the
first batch of samples.
---
 js/publish/src/audio/encoder.ts | 57 +++++++++++++++++++++++----------
 1 file changed, 40 insertions(+), 17 deletions(-)

diff --git a/js/publish/src/audio/encoder.ts b/js/publish/src/audio/encoder.ts
index c7dcb138a..c4a9a5ed3 100644
--- a/js/publish/src/audio/encoder.ts
+++ b/js/publish/src/audio/encoder.ts
@@ -66,7 +66,6 @@ export class Encoder {
 		this.groupDuration = props?.groupDuration ?? (100 as Time.Milli); // Default is a group every 100ms
 
 		this.#signals.run(this.#runSource.bind(this));
-		this.#signals.run(this.#runConfig.bind(this));
 		this.#signals.run(this.#runGain.bind(this));
 		this.#signals.run(this.#runCatalog.bind(this));
 	}
@@ -102,14 +101,29 @@ export class Encoder {
 			await context.audioWorklet.addModule(CaptureWorklet);
 			if (context.state === "closed") return;
 
+			const channelCount = settings.channelCount ?? root.channelCount;
 			const worklet = new AudioWorkletNode(context, "capture", {
 				numberOfInputs: 1,
 				numberOfOutputs: 0,
-				channelCount: settings.channelCount ?? root.channelCount,
+				channelCount,
 			});
 
 			effect.set(this.#worklet, worklet);
 
+			// The information about channels count can be unreliable on different platforms (Apple's safari).
+			// Try to get the first audio frame and only then create the configuration.
+			worklet.port.onmessage = ({ data }: { data: Capture.AudioFrame }) => {
+				const channelCount = data.channels.length;
+				if (!channelCount) return;
+
+				this.#config.set(this.#createConfig(worklet, channelCount));
+				worklet.port.onmessage = null;
+			};
+			effect.cleanup(() => {
+				worklet.port.onmessage = null;
+				this.#config.set(undefined);
+			});
+
 			gain.connect(worklet);
 			effect.cleanup(() => worklet.disconnect());
 
@@ -118,23 +132,17 @@ export class Encoder {
 		});
 	}
 
-	#runConfig(effect: Effect): void {
-		const values = effect.getAll([this.source, this.#worklet]);
-		if (!values) return;
-		const [_source, worklet] = values;
-
-		const config = {
+	#createConfig(worklet: AudioWorkletNode, channelCount: number): Catalog.AudioConfig {
+		return {
 			codec: "opus",
 			sampleRate: Catalog.u53(worklet.context.sampleRate),
-			numberOfChannels: Catalog.u53(worklet.channelCount),
-			bitrate: Catalog.u53(worklet.channelCount * 32_000),
+			numberOfChannels: Catalog.u53(channelCount),
+			bitrate: Catalog.u53(channelCount * 32_000),
 			container: { kind: "legacy" } as const,
 			// TODO parse the actual frame duration instead of assuming 20ms.
 			// Opus supports 2.5–60ms but 20ms is the real-time default.
 			jitter: Catalog.u53(20),
 		};
-
-		effect.set(this.#config, config);
 	}
 
 	#runGain(effect: Effect): void {
@@ -153,9 +161,9 @@ export class Encoder {
 	}
 
 	serve(track: Moq.Track, effect: Effect): void {
-		const values = effect.getAll([this.enabled, this.#worklet, this.#config]);
+		const values = effect.getAll([this.enabled, this.#worklet]);
 		if (!values) return;
-		const [_, worklet, config] = values;
+		const [_, worklet] = values;
 
 		effect.set(this.active, true, false);
 
@@ -190,11 +198,26 @@ export class Encoder {
 			});
 			effect.cleanup(() => encoder.close());
 
-			console.debug("encoding audio", config);
-			encoder.configure(config);
+			let config = this.#config.peek();
+			if (config) {
+				console.debug("encoding audio", config);
+				encoder.configure(config);
+			}
 
 			worklet.port.onmessage = ({ data }: { data: Capture.AudioFrame }) => {
-				const channels = data.channels.slice(0, worklet.channelCount);
+				const channelCount = data.channels.length;
+				if (!channelCount) return;
+
+				if (!config || channelCount !== config.numberOfChannels) {
+					config = this.#createConfig(worklet, channelCount);
+					this.#config.set(config);
+					lastKeyframe = undefined;
+
+					console.debug("encoding audio", config);
+					encoder.configure(config);
+				}
+
+				const channels = data.channels;
 				const joinedLength = channels.reduce((a, b) => a + b.length, 0);
 				const joined = new Float32Array(joinedLength);